Chapter 2 – Elementary Cryptography 1 SHIRAJ MOHAMED M | MIS UNIT.

Chapter 2 – Elementary Cryptography

1SHIRAJ MOHAMED M | MIS UNIT

Concepts of encryption Cryptanalysis Symmetric (secret key) Encryption Asymmetric (public key) Encryption Key exchange protocols and certificates Digital Signatures Cryptographic hash functions


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Sender (S), Recipient (R), Transmission media (T)

Interceptor / intruder (O) (availability) O might block message from reaching R O might intercept message

(confidentiality) O might modify message (integrity) O might fabricate an authentic-looking

message (integrity)


Encryption – process of encoding a message

Decryption – transforming encoded message back to normal

Encrypt – encode , encipher Decrypt – decode, decipher Cryptosystem – system for encryption and

decryption Plaintext – original form of message Ciphertext – encoded form of message


Algorithms – rules for encryption and decryption Key – value used to encrypt message C = E(K, P) where P=plaintext, K = key, E = encryption algorithms,

and C = ciphertext Symmetric encryption P = D(K, E(K,P)) Asymmetric encryption P = D(KD, E(KE,P)) Keyless cipher Cryptography (hidden writing) – uses encryption to hide

message Cryptanalysis – attempts to find meanings in encrypted

messages Cryptology – study of encryption and decryption


Sometimes the encryption and decryption keys are the same

P = D (K, E (K, P))

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where P=plaintext, K = key, E = encryption algorithms, and C = ciphertext

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At other times, encryption and decryption keys come in pairs

Decryption key, KD

Encryption key KE

P = D (KD, E (KE, P))

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where P=plaintext, K = key, E = encryption algorithms, and C = ciphertext


Cryptography (secret writing) is the strongest tool for controlling against many kinds of security threats.

Well-disguised data cannot be read, modified, or fabricated easily. Cryptography is rooted in higher mathematics: group and field theory, computational complexity, and even real analysis, not to mention probability and statistics. Fortunately, it is not necessary to understand the underlying mathematics to be able to use cryptography.


Attempt to break a single message Attempt to recognize patterns in

encrypted messages Attempt to infer some meaning without

breaking the encryption Attempt to realize the key Attempt to find weaknesses in the

implementation or environment of use of encryption

Attempt to find general weaknesses in an encryption algorithm


CryptographerA cryptographer works on behalf of a legitimate

sender or receiver

CryptanalystA cryptanalyst works on behalf of an unauthorized

interceptor


An encryption algorithm is called breakable when, given enough time and data, an analyst can determine the algorithm

May be impractical A 25-character message of just uppercase

letters has 2625 (1035) possible decipherments. A computer performing 1010 operations/sec would take 1011 years



we use the convention;plaintext is written in UPPERCASE letters, and ciphertext is in lowercase letters

LASANTHA11018131970

A + 3 = D N - 1 = ? C+10 = ? S + 9 = ? X + 4 = ?


Substitution – one or more characters are replaced with another

Transpositions (permutations) – order of characters is rearranged

Hybrid – combinations of the two types


This technique is called a monoalphabetic cipher or simple substitution

A substitution is an acceptable way of encrypting text


Each letter is translated a fixed number of positions in the alphabet

Ci = E(pi) = pi + 3 (Caesar used a shift of 3)

Easy to perform; easy to break Look for double letters and then use

common words with double letters


Use a key to scramble the letters A B C D E F G H I J K L M N O … c i p h e r s a b d f g j k l …

Rearrange using a fixed distance between letters (e.g. every 3rd)

A B C D E F G H I J K L M N O … a d g j m p s v y b e h k n r …


Substitution encryption algorithms can be performed by direct lookup in tables.

An important issue in using any cryptosystem is the time it takes to turn plaintext into ciphertext, and vice versa.


The techniques described for breaking the Caesar cipher can also be used on other substitution ciphers

Look for short words, words with repeated patterns, common first and last letters

Can use our knowledge of language Look at frequency distributions Could reduce time to hours Nature and context of the text being

analyzed


The pad consists of a large number of pages where each page contains a non-repeating key

The sender would write the keys above the message (e.g. a 300 character message would require 30 pages of 10 character keys)

The message is scrambled using a Vigenere tableau built from the message and key

Problem is synchronizing the receiver’s pad with the senders pad



I am, I exist, that is certain.

uaopm kmkvt unhbl jmed

One-time pad consists of an arbitrary long non-repeating sequence of numbers that are combined with the plaintext

Each plaintext character is represented by its numeric equivalent and is added to one of the random numbers. The ciphertext character is computed from the sum mod 26

Repeated characters are typically represented by different ciphertext characters


Uses a passage from a book to form the letters at the top of a Vigenere Tableau

Computes ciphertext character by taking the intersection of the plaintext character and corresponding character at that position from the book passage

Relatively easy to break using frequency distributions


Columnar Transposition rearranging plaintext message into columns and then reading it row by row

Transposition algorithms require a constant amount of time per character and are (n) algorithms, but space required to store results and delay in waiting for all characters to be read are dependent on the size of the plaintext


THIS IS A MESSAGE TO SHOW HOW A COLUMNAR TRANSPOSITION WORKS

tssoh oaniw haaso lrsto imghw utpir seeoa mrook istwc nasns


If the message length is not a multiple of the length of a row, the last columns will be one or more letters short. When this happens, we sometimes use an infrequent letter, such as X, to fill in any short columns


Compute letter frequencies of ciphertext; if appear with normal frequency, then assume a transposition algorithm was used


The amount of secrecy needed should determine the amount of labor appropriate for the encryption and decryption

The set of keys and the enciphering algorithm should be free from complexity

The implementation of the process should be as simple as possible

Errors in ciphering should not propagate and cause corruption of further information in the message

The size of the enciphered text should be no larger than the text of the original message


Most of the ciphers we have presented so far are stream ciphers (exception is the columnar transposition cipher)

convert one symbol of plaintext immediately into a symbol of ciphertext


Skipping a character in the key during encryption


A block cipher encrypts a group of plaintext symbols as one blockEg: columnar transposition


Advantages and disadvantages of stream and block encryption algorithms


The interceptor should not be able to predict what will happen to the ciphertext by changing one character in the plaintext

The goal of substitution is confusion


The cipher should also spread the information from the plaintext over the entire ciphertext so that changes in the plaintext affect many parts of the ciphertext


Charles P. Pfleeger, (2005) "Security in Computing (Fourth Edition)", Prentic-Hall International, Inc.


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thank you


Chapter 2 – Elementary Cryptography 1 SHIRAJ MOHAMED M | MIS UNIT.

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